In a phased array ultrasound imaging system, the ultrasound transducer includes an array of transducer elements. The system includes n parallel channels, each having a transmitter and a receiver connected to one of the transducer array elements. Each transmitter outputs an ultrasound pulse through the transducer element into an object being imaged, typically the human body. The transmitted ultrasound energy is steered and focused by applying appropriate delays to the pulses transmitted from each array element so that the transmitted energy adds constructively at a desired point. The pulse is partially reflected back to the transducer array by various structures and tissues in The body.
Steering and focusing of the received ultrasound energy is effected in a reverse manner. The reflected ultrasound energy from an object or structure arrives at the array elements at different times. The received signals are amplified, delayed and then summed in a receive beamformer. The delay for each element is selected such that the received beam is focused at a desired point. The delays may be varied dynamically so as to focus the beam at progressively increasing depths, or ranges, as the ultrasound energy is received. The transmitted beam is scanned over a region of the body, and the signals generated by the beamformer are processed to produce an image of the region.
One important consideration in ultrasound imaging is the image sequence rate, or frame rate. The frame rate is limited by the speed of propagation of the ultrasound energy in the human body, the depth being examined and the number of scan lines used to form the image. The frame rate is particularly critical for color Doppler imaging of blood flow and for producing high resolution images.
One approach to increasing the frame rate is to receive beams from more than one direction at the same time within the spread of the transmitted pattern. In another high frame rate approach, several widely spaced receive beams are processed at the same time. A third class of applications for multiple receive beams involves using a "normal" transmit pattern and many receive beams in order to approximately calculate a complete synthetic aperture data set, from which several useful image enhancements can be calculated. In prior art systems, multiple receive beams are formed by multiple beamformers operating in parallel. However, because of the large amount of circuitry required for each beamformer, this approach is very expensive and impractical.
U.S. Pat. No. 4,644,795, issued Feb. 24, 1987 to Augustine, discloses a multiline ultrasonic beamformer which utilizes a sin(.pi.x)/.pi.x transmit excitation, and the received signals are applied to parallel delay paths.
U.S. Pat. No. 4,790,320, issued Dec. 13, 1988 to Perten et al discloses an ultrasound imaging system wherein delay processors, such as dual port RAMS, include incremental delays in order to perform parallel beamforming simultaneously.
U.S. Pat. No. 4,886,069, issued Dec. 12, 1989 to O'Donnell, discloses a technique for obtaining return signals from M different beam directions simultaneously by demodulating return signals and rotating the phases of the received signals.
U.S. Pat. No. 4,622,634, issued Nov. 11, 1986 to Fidel, discloses a system for parallel processing of ultrasound vectors wherein first and second memories are alternately loaded with vector information. The vector information is read out from the memories at different times.
U.S. Pat. No. 4,893,283, issued Jan. 9, 1990 to Pesque, discloses an ultrasound system wherein the transmitter transmits M beams sequentially during a very short time period, and the receiver simultaneously receives the echoes of the M beams.
U.S. Pat. No. 5,121,361, issued Jun. 9, 1992 to Harrison, Jr. et al, discloses a programmable beamformer including first and second programmable beam focusing modules which operate alternately to focus in different zones. It appears that only one beam is formed.
U.S. Pat. No. 4,252,026, issued Feb. 24, 1981 to Robinson, discloses an ultrasonic system wherein a plurality of beamforming circuits provide receive beams for each pulse transmitted.
U.S. Pat. No. 4,173,007, issued Oct. 30, 1979 to McKeighen et al, discloses an ultrasound imaging system using a memory with separate read and write capabilities to produce a dynamically variable delay. The delay can be varied by modifying the write or the read address pointer.